WiFiClient::peek should return -1 instead of 0

Fixes: https://github.com/espressif/arduino-esp32/issues/329

libraries/WiFi/src/WiFiClient.h

@@ -49,7 +49,7 @@ public:
     int read(uint8_t *buf, size_t size);
     int peek()
     {
-        return 0;
+        return -1;
     }
     void flush();
     void stop();